The present invention relates generally to inject printing mechanisms, and more particularly to a system for monitoring a pressure wave developed in the surrounding ambient environment during the process of inkjet droplet formation. The system uses the pressure wave information to determine current levels of printhead performance, and if required, the system then adjusts the print routine, services the printhead, or alerts an operator, for instance, that an inkjet cartridge is nearly empty.
Inkjet printing mechanisms use cartridges, often called xe2x80x9cpensxe2x80x9d, which shoot drops of liquid colorant, referred to generally herein as xe2x80x9cink,xe2x80x9d onto a page. Each pen has a printhead formed with very small, pin-hole-sized nozzles through which the ink drops are fired. To print an image, the printhead is propelled back and forth across the page, shooting drops of ink in a desired pattern as it moves. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanism are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization or firing chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text).
To clean and protect the printhead, typically a xe2x80x9cservice stationxe2x80x9d mechanism is mounted within the print chassis so the printhead can be moved over the station for servicing and maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as xe2x80x9cspitting,xe2x80x9d with this non-image producing waste ink being collected in a xe2x80x9cspittoonxe2x80x9d reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the printhead.
To improve the clarity and contrast of the printed image, recent research has focused on improving the ink itself. To provide faster drying, more waterfast printing with darker blacks and more vivid colors, pigment based inks have been developed. These pigments based inks have a higher solid content than the earlier dye based inks, which results in a higher optical density for the new inks. Both types of ink dry quickly, which allows inkjet printing mechanisms to use plain paper. Unfortunately, the combination of small nozzles and quick drying ink leaves the printheads susceptible to clogging, not only from dried ink and minute dust particles or paper fibers, but also from the solid within the new inks themselves. Partially or completely blocks nozzles can lead to either missing or misdirected drops on the print media, either of which degrades the print quality. Besides merely forcing clogs out of the nozzles, spitting also heats the ink near the nozzles, which decreases the ink viscosity and assists in dissolving ink clogs. Spitting to clear the nozzles becomes even more important when using pigment based inks, because the higher solids content contributes to the clogging problem more than the earlier dye based inks.
The pen body may serve as an ink containment reservoir that protects the ink from evaporation and holds the ink so it does not leak or drool from the nozzles, Ink leakage is prevented using a force known as xe2x80x9cbackpressure,xe2x80x9d which is provided by the ink containment system. Desired backpressure levels may be obtained using various types of pen body designs, such as resilient bladder designs, spring-bag designs, and foam-based designs.
To maintain reliability of the inkjet printing mechanism during operation, it would be helpful to have advanced warning for an operator as to when the ink level in a cartridge is getting low. This would allow an operator to procure a fresh inkjet cartridge before the one in use is completely empty. If the cartridge is refillable, an early warning would allow an operator to replenish the ink supply before the pen is dry-fired. Dry-firing an inkjet cartridge when empty may cause permanent damage to the printhead by overheating the resistive heater elements, causing the resistors to burn out.
A variety of solutions have been proposed for monitoring the level of ink within inkjet cartridges, with many incorporating measuring devices inside the cartridge. For example, several mechanism devices have been proposed to determine when the ink supply falls below a predetermined level. One system uses a ball check valve within an ink bag to interrupt ink flow when the pen is nearly empty. Unfortunately, this system has no early warning capability and it may abruptly interrupt a printing job when a certain level of ink is reached.
Other earlier ink level monitoring systems kept a running count of the number of drops fired, which worked well until cartridges were exchanged. Unfortunately, these drop counting systems had no way of determining whether a new or a partially used cartridge was installed, so they failed to detect upcoming empty conditions for the partially used cartridges. Several more sophisticated detection systems have been devised, based upon measuring printhead temperature changes after spitting specific amounts of ink into the spittoon. These temperature monitoring systems were slow to use, and they wasted ink that could otherwise have been used for printing. Other systems have been proposed using specially designed nozzles which are more sensitive to changes in the ink reservoir backpressure than the remaining nozzles, with these backpressure changes indicating ink depletion.
In operating an inkjet printing mechanism, it would be helpful to provide feedback to a print controller, such as a printer driver residing in an on-board microprocessor and/or in the host computer, as to whether or not the printhead nozzles are firing as instructed. This information would be useful to determine whether a nozzle had become clogged and required purging or spitting to clear the blockage. This information would streamline the spitting process and conserve ink because only the clogged nozzle(s) would be spit to clear the blockage. Moreover, if damaged nozzles or heating elements could be detected, then other nozzles may be substituted in the firing scheme to compensate for the damaged nozzles. Feedback as to nozzle firing could also be used to test the electro-mechanical interconnect between a replaceable inkjet cartridge and the printing mechanism. Over time, this interconnect may be contaminated with ink, interrupting the electrical connections. When this happens, it would be desirable to notify the user to clean the interconnect.
As a manufacturing quality control check, it would also be desirable to monitor nozzle performance, for instance, to verify correct nozzle-to-nozzle alignment. It would also be helpful to check for any nozzle telecentricity, that is, any lack of perpendicularly of the orifice hole through the nozzle plate relative to the plate surface. Another important feature to monitor would be nozzle directionality, that is whether a nozzle was firing at an angle other than perpendicular to the orifice plate and/or to the media.
It would also be useful to determine from merely firing ink droplets at media, what type of media was inserted into the printing mechanism, such as plain paper, glossy high-quality paper, or transparencies. This information would then allow the printer controller to adjust the print mode to correspond to the type of media in use. One desirable energy saving would be to use only the minimum xe2x80x9cturn-onxe2x80x9d energy required to eject ink from each of the nozzles. Using only the minimum amount of firing energy would extend printhead life by minimizing overheating of the heaters in the printhead. This minimum firing energy operation could be accomplished by providing drop feedback to the printer controller.
In the past, some inkjet printing mechanism have detected drops using optical means. For example, one system measured the change in drop volume for a given firing temperature by firing smaller and smaller droplets until the drops could no longer be seen by the optical detector. Unfortunately, the target drop volume has decreased in newer inkjet cartridges, for example, some droplets are now on the order of 30 picoliters. These small droplets require precise positioning of such an optical drop detector, which is difficult to implement consistently and reliably in production printing mechanisms. Other drop detect systems addressed the nozzle-to-nozzle and the printhead-to-printhead alignment issues by printing several test patterns, from which a user then selects the best pattern or compares the test pattern to a reference pattern in the instruction manual. In these visual tagging systems, the printer controller or driver then adjusts the printing mode to an optimum level that corresponds the pattern selected by the user. Another visual system uses a tab connected to the internal spring-bag reservoir to retract the tab as the pen empties, giving the user a visual ink level indicator on the pen body. Unfortunately, these visual tagging systems required user intervention or judgment, so they were not automatic or xe2x80x9ctransparentxe2x80x9d to the user in operation.
In multi-printhead systems, such as those carrying two, three, four or more cartridges, it would also be desirable to have an automatic method of monitoring the pen-to-pen alignment. This pen-to-pen alignment could then be used to adjust the firing sequence of the nozzles to compensate for any misalignment of the pens. Pen-to-pen misalignment may be caused by improper seating within the pen carriage, or an accumulation of tolerance variations within a specific pen body and printhead of a particular cartridge. Pen-to-pen misalignment may also be caused by an accumulation of tolerance variations within a specific printer carriage which holds the cartridges.
Thus, a need exists for a system to provide inkjet droplet information to the printing mechanism controller. This information would allow the controller to respond by adjusting droplet formation or print modes, servicing the pen, or alerting the operator of a particular condition, for instance, that an inkjet cartridge is nearly empty.
According to one aspect of the present invention, an ultrasonic monitoring method of operating an inkjet printing mechanism is provided for a printing mechanism having an inkjet printhead installed therein, with the printhead having plural nozzles. The method includes the steps of applying an enabling signal to a selected nozzle of the inkjet printhead, and normally generating a pressure wave in response to the applying step. The method also includes the steps of ultrasonically detecting the pressure wave emitted by the selected nozzle during the generating step, and then responding to the detecting step.
According to another aspect of the invention, an inkjet printing mechanism is provided as including an inkjet printhead with plural nozzles that each normally, in response to an enabling signal, eject ink therethrough and generate a pressure wave comprising both audio and ultrasonic frequency components. The printing mechanism has an ultrasonic pressure wave sensor located to detect the ultrasonic pressure waves normally generated by the plural nozzles and in response thereto, the sensor generates a wave signal. The printing mechanism also has a controller that responds to the wave signal by generating an action signal.
According to an additional aspect of the invention, a method of monitoring the performance of an inkjet printhead having plural nozzles is provided. The method includes the steps of applying an enabling signal to a selected nozzle of the inkjet printhead, and normally generating a pressure wave in response to the applying step. In a detecting step, the pressure wave emitted by the selected nozzle during the generating step is detected from plural locations, and in response to the detected pressure wave, a wave signal is generated from each of the plural locations. In an analyzing step, the wave signal from each of the plural locations is analyzed to determine performance of the selected nozzle.
In a further aspect of the invention, an inkjet printhead is provided for an inkjet printing mechanism that generates plural firing signals. The printhead has an ink reservoir holding a supply of ink and an orifice plate defining plural nozzles extending therethrough. An ink ejection mechanism fluidicly couples the ink reservoir to the orifice plate nozzles. The ink ejection mechanism comprises plural in ejection chambers each responsive to at least one of the plural firing signals to normally eject ink through an associated one of the plural nozzles. An accelerometer mechanism is located adjacent to the ink ejection mechanism to detect a pressure wave normally generated in response to at least one of the plural firing signals, and to generate a wave signal in response thereto.
An overall goal of the present invention is to provide an inkjet droplet formation monitoring system to generate information that may be used to determine current levels of performance, which is then used by the printer controller to optimize performance. This information may be used for a variety of other purposes, such as to give an early warning before an inkjet cartridge is completely empty, allowing an operator to refill, replace or service the cartridge.
An additional goal of the present invention is to provide a monitoring system that may be used during printhead manufacture to verify the quality of printhead performance.
Another goal of the present invention is to provide a monitoring system that may be used with any type of inkjet printhead, and to provide a special printhead that has a sensor integrally formed therein.
A further goal of the present invention is to provide an inkjet droplet formation monitoring system, as well as a printing mechanism and a method which optimizes the print quality of an image in response to this monitoring.